Thin film transistors have been fabricated incorporating organic semiconducting materials, such as pentacene, polythieneylenevinylene, thiophene oligomers, benzothiophene dimers, and polyacetylenes. Organic materials can also be used to form the other components of the transistor, such as the conducting layers that form the gate, source, and drain electrodes, and the insulating layers that form the dielectric.
Transistors made in whole or in part of organic materials may be less expensive and easier to manufacture than traditional transistors. While the same component densities as silicon transistors have not yet been achieved, the low cost of organic transistors means that they can be used in applications where high density is not required and traditional transistors are not economical. For example, organic transistors could be used in inexpensive or disposable items, such as electronic paper, posters and books, smart cards, toys, appliances and electronic bar codes for product identification. Organic transistors can also be flexible, which is advantageous in certain applications. For example, flexible transistor arrays can be used in flexible electrophoretic displays, PLEDs and liquid crystal displays (LCDs) for computers, laptops and televisions. While the savings in fabrication costs are significant, further decreases in the fabrication costs of organic transistors would be advantageous.
Organic materials can be applied to a portion of a transistor by spin coating, casting, printing or other methods. Some organic materials can also be applied by physical vapor deposition processes. An electroactive polymer precursor can also be applied and converted to a polymer, typically by heat. Using a mask can provide direct patterning during deposition. If a photoresist is used during deposition, wet chemical etching after deposition is necessary, which may result in severe degradation of the organic semiconductor. While easier and less expensive than the fabrication techniques required by silicon based transistors, such methods are still complex, slow, lack sufficient resolution, expose the device to deleterious heat and chemical processes, and are more expensive than necessary.
Fabricating organic transistors completely by printing techniques offers the potential for further cost reductions. F. Gamier et al., “All-Polymer Field-Effect Transistor Realized by Printing Techniques”, Science, Vol. 265, 16 Sep. 1994, pp.1684-1686, disclose the formation of a transistor by deposition of a conducting graphite-based polymer ink through masks to form gate, source and drain electrodes. Semiconducting material of α,ω-di(hexyl)sexithiophene was deposited over the source and drain by flash evaporation.
Z. Bao, et al., “High-Performance Plastic Transistors Fabricated by Printing Techniques,” Chem. Mater. 1997, 9, 1299-1301, disclose the production of a high-performance transistor in which the essential components were printed directly on an indium tin oxide (ITO) coated plastic substrate. Masks were used to form the printed patterns of the transistor components.
Ink-jet printing has also been used to apply organic semiconducting material. See U.S. Pat. No. 6,087,196; EP 0880303A1; WO 99/66483; and WO 99/43031. While facilitating the fabrication process, screen-printing and ink-jet printing do not provide sufficient resolution for certain applications. In addition, it is also difficult to control the flatness and uniformity of the final film in ink-jet printing processes.
Thermal transfer processes are well known in applications such as color proofing. Such thermal transfer processes include, for example, dye sublimation, dye transfer, melt transfer, and ablative material transfer, and typically use a laser to induce the imagewise thermal transfer of material. These processes have been described in U.K. 2,083,726; U.S. Pat. Nos. 4,942,141; 5,019,549; 4,948,776; 5,156,938; 5,171,650; and 4,643,917.
Laser-induced thermal transfer processes typically use a donor element, including a layer of material to be transferred (“transfer layer”), and a receiver element, including a surface for receiving the transferred material. Either the substrate of the donor element or the receiver element is transparent, or both are transparent. The donor element and receiver element are brought into close proximity or into contact with each other and selectively exposed to laser radiation, usually by an infrared laser. Heat is generated in the exposed portions of the transfer layer, causing the transfer of those portions of the transfer layer onto the surface of the receiver element. If the material of the transfer layer does not absorb the incoming laser radiation, the donor element must include a heating layer adjacent to the transfer layer. An ejection layer of a vaporizable polymeric material, which decomposes into gaseous molecules when heated, may be also provided between the heating layer and the donor support. Decomposition of the ejection layer provides additional force for propelling the exposed portions of the transfer layer onto the receiver element.
In one laser-induced digital thermal transfer process, the exposure takes place only in a small, selected region of the assembly at a time, so that transfer of material from the donor element to the receiver element can be built up one pixel at a time. Computer control facilitates the transfer at high speed and high resolution. Alternatively, in an analog process, the entire assembly is irradiated and a mask is used to selectively expose desired portions of the thermally imageable layer (U.S. Pat. No. 5,937,272).
Laser-induced thermal transfer processes are generally faster and less expensive than the coating, deposition and patterning processes described above and allow the patterning of features at high resolution. Although printing an item in a printing press offers high speed, large-area printing and high resolution, a laser-induced thermal process has the additional advantage of not requiring solvent compatibility of layers printed sequentially, thereby broadening the range of useable materials. However, direct thermal printing of extremely thin films of semiconductors or light-emitting organic materials that are fragile and sensitive to perturbations (e.g., large temperature gradients, humidity, pressure or mechanical stress) has not been achieved. Attempts to thermal transfer materials such as pentacene, fluorinated copper phthalocyanine, or organic light-emitting materials usually results in severe degradation and/or partial vaporization of the materials.
There is a need for thermal transfer processes, particularly laser-induced thermal transfer processes, that can be used in the application and patterning of organic semiconducting materials for the fabrication of organic transistors and other organic electronic devices, and of light-emitting materials for the fabrication of light-emitting devices such as displays.